You have no idea what can happen in 15 years, and you can‘t be sure that Steam still exist, or if at a certain point Windows Games are obsolete and just dissapear from the steam library just like stuff for older consoles like the original XBox.
15 Years ago Nokia was the most successful cellphone producer on earth, just look where they are now. Really, you don‘t know nothing, so just be sure that you can‘t assume that Steam exist in 15 years, at lease in it‘s current form.
Discs on the other hand can exist that long. My Doom v1.1 3.5“ floppies are still in an excellent condition. I guess you can calculate how old they are on your own.
And for your question regarding the renderer used for Ion Maiden, why you don‘t just look at the eduke32 website?
Polymost is replaced by Polymer since ages.

I checked both files, the infos on the OGDB are correct. Both WADs have the BFG edition cuts. But these are not the BFG edition WADS.
These are the hashes:
DOOM.WAD
SHA-1: 117015379C529573510BE08CF59810AA10BB934E
SHA-256: 76A22247D76EE9710595F7EE2D8DDED2CE9785FB49287A9AC6348544B858E6F9
DOOM2.WAD
SHA-1: 97573AAF26957099ED45E61D81A0A1A3
SHA-256: F1B6BA94352D53F646B67C01D2DA88C5C40E3179
I just compared them to the BFG IWADs, had no chance to compare them with the GOG release IWADs atm, for i had no windows machine to install them. Will check that maybe later.

In germany there was a release on CD in a gaming mag in 2016.
http://ogdb.eu/index.php?section=compilation&compid=4266
It contains Ultimate Doom and Doom 2 on disc. Only thing is, you need a password to install, which you could just get for a limited time. But according to the password i got, and the process to get it, i think there is only one password for all.

Are you ready for some nostalgia? Look what i found on one of my hard disks.
In the third trailer is a short sequence, which seems to take place in the map, which is to be seen in one of the screenshots. ;-)

When released in '93, Doom was seen as a game with an horror atmosphere too. Even the music in parts, Dark Halls, Suspense or Sign of Evil are in fact not so metal as the soundtrack is often seen. PSX Doom and Doom64 added much to this, using improved tech.

Same here, a friend of mine brought his PC to my apartment. We connected them with a DE-9 null modem cable, copied all necessary files over this connection via Norton Commander, and played dozens of hours Doom Coop.